This invention is generally directed to an improved process for fixing toner images, and more specifically the present invention is directed to a process for fixing developed polyamide toner images to suitable substrates by subjecting the image to cold pressure fixing, followed by radiant assist fixing. There is thus provided in accordance with the process of the present invention an improved method for fixing developed electrostatic latent images to appropriate substrates such as paper, which process allows the use of low pressures, and low radiant energy sources, thereby resulting in permanently fixed images possessing excellent solid area coverage.
The development of electrostatographic images, and in particular xerographic images with developer compositions containing toner materials is well known. In these systems an electrostatic latent image is formed on a photoconductive member, and the image is developed with a toner composition comprised of resin particles, and pigment particles. Subsequently, the developed image is transferred to a suitable substrate wherein fixing is generally accomplished by heat. Thus, final toner images are produced by heating the toner particles to a temperature at which these particles begin to flow in order to effect heat fusing of the particles to a support substrate such as paper. Generally, one disadvantage of the heat fixing process is that substantial energy is needed, and further the imaging device involved, such as a xerographic imaging machine, may in some instances require sufficient warm-up time in order to enable the toner image to be properly fused. An example of such a heat fusing system is described in U.S. Pat. No. 4,256,818, wherein there is disclosed the heating of the developed toner image for the purpose of causing the resins contained therein to at least partially melt and become adhered to the photoconductive imaging member, followed by the application of pressure to the toner with heating, such as the use of a heated roller. There is further disclosed in this patent a solvent vapor fusing process wherein the resin component of the toner is partially dissolved. However, it is known that prolonged heating of toner materials at high temperatures suffers from a number of defficiencies, for example, the paper to which the toned image is being fixed can be ignited or charred. Furthermore, the introduction of excessive amounts of heat into the xerographic imaging system can cause damage to other machine parts. While pressure fusing eliminates some of the disadvantages inherent in lengthy heat fusing cycles, pressure fusing alone has other disadvantages including the requirement for high pressures, and specially formulated toner compositions. Additionally, pressure fusing alone generally does not result in an image which is well fused to the paper substrate.
Cold pressure fusing processes nevertheless have a number of advantages primarily relating to the requirement for less energy as the toner compositions involved can be fixed, for example, at room temperature. Many prior art toner compositions selected for cold pressure fixing systems have been known to suffer from a number of deficiencies, for example, these toner compositions must usually be fused under high pressures, which pressures have a tendancy to severely disrupt the toner fusing characteristics of the compositions selected. This can result in images of low resolution, or no images whatsoever, and in some of these systems substantial image smearing has been noticed because of the high pressures required. While attempts have been made to improve toner compositions for cold pressure fixing systems, these compositions in many instances have a number of undesirable characteristics, including agglomeration of the toner particles at room temperature, insufficient flowability of these particles under high pressures, lack of adhesion of the toner particles to the support substrate, such as paper, unsuitable blocking temperatures, and an insufficient brittleness to allow preparation of such materials by, for example, known commercial jetting methods, or known fluid energy milling processes.
There is disclosed in U.S. Pat. No. 3,928,656, a pressure fixable toner comprised of a weakly cross-linked amorphous polymer, the cross-linked bonds of which are disrupted and/or broken by the application of pressure, and wherein the sufficiently soft polymeric material selected can be fixed by pressure. It is disclosed in this patent that the resinous materials include a weakly cross-linked amorphous polymer having a glass transition temperature of greater than about minus 20 degrees centigrade. Apparently the crosslinks of the polymer which are shear sensitive, can be temporarily disrupted, and are broken by the application of pressure resulting in a polymer which has the properties of an uncrosslinked polymer. When the pressure is released the polymer reverts to its crosslinked state. Accordingly, such a toner composition is capable of being fixed to a support medium in image configuration by the application of pressure, which pressure is generally provided by pressing the substrate material with the toner image contained thereon between a pair of polished metal rollers that are in contact with one another under a specified pressure. In general the metal rollers exert a pressure of form about 10 to about 600 pounds per linear inch, and preferably a pressure of from about 50 to about 400 pounds per linear inch, which pressure is calculated by dividing the total applied force by the length of the roll.
Additionally, disclosed in a co-pending application are pressure sensitive toner compositions comprised of a blend of two or more polymers selected from the group consisting of a blend of a polymer of polystyrene-co-stearylmethacrylate, and poly(octadecylvinylether-co-maleic anhydride); and polyisobutylmethacrylate polymers and poly(octadecylvinylether-co-maliec anhydride). The toner compositions described in this co-pending application exhibit sufficient flowability to allow proper development to occur, do not agglomerate or block at temperature of 120 degrees Faherenheit, and have sufficient adhesion properties to allow such compositions to be permanently bonded to suitable substrates such as plain bond paper. These toners when selected for the development of electrostatic latent images formed, for example, on selenium imaging members, were fused to plain bond paper with cold pressure rollers maintained at a pressure of from about 200 to 500 pounds per linear inch.
Nevertheless there continues to be a need for improved processes for fixing toner images to suitable substrates. Additionally, there continues to be a need for improved fixing methods wherein the resulting developed images can be fixed to suitable substrates with low energy inputs. Additionally, there continues to be a need for improved processes wherein various toner compositions can be fixed to suitable substrates by a combination of pressure fixing and radiant heat fixing steps. Moreover, there continues to be a need for an imaging process wherein the resulting images are of high resolution.